Conventional gas turbine engines operate at high temperatures and therefore, many of the systems within the engine are formed from metals capable of withstanding the high temperature environments. For example, gas turbine systems often include ring segments that are stationary gas turbine components located between stationary vane segments at the tip of a rotating turbine blade or airfoil. Ring segments are exposed to high temperatures and high velocity combustion gases and are typically made from metal. While the metal is capable of withstanding the operating temperatures, the metal is often cooled to enhance the usable life of the ring segments. Many current ring segment designs use a metal ring segment attached either directly to a metal casing or support structure or attached to metal isolation rings that are attached to the metal casing or support structure. More recently, firing and/or operating temperatures of turbine systems have increased to improve engine performance. As a result, the ring segments have required more and more cooling to prevent overheating and premature failure. Even with thermal barrier coatings, active cooling is still necessary.
Ceramic materials, such as ceramic matrix composites, have higher temperature capabilities than metal alloys and therefore, do not require the same amount of cooling, resulting in a cooling air savings. Prior art ring segments made from CMC materials rely on shell-type structures with hooks or similar attachment features for carrying internal pressure loads. U.S. Pat. No. 6,113,349 and U.S. Pat. No. 6,315,519 illustrate ring segments with C-shaped hook attachments. Conventional ceramic matrix components are formed from layers of woven fibers positioned in planes and layers substantially parallel to the inner sealing surface of the ring segments. For cooled components, internal pressurization would load these attachment hooks in such a way as to cause high interlaminar tensile stresses. Other out-of-plane features common in laminated structures, such as T-joints, are also subject to high interlaminar stresses when loaded. One of the limitations of laminated ceramic matrix composite (CMC) materials, whether oxide or non-oxide based, is that their strength properties are not generally uniform in all directions (e.g. the interlaminar tensile strength is generally less than about 5% of the in-plane strength). Nonuniform fiber perform compaction in complex shapes and anisotropic shrinkage of matrix and fibers results in delamination defects in small radius corners and tightly curved sections, further reducing the already-low interlaminar properties. A further limitation of shell-type CMC construction is that the through-thickness thermal conductivity is lower than the in-plane conductivity, particularly for oxide based CMC's. Many applications of CMC require cooling, preferably convective cooling on one side, removing heat by through-thickness conduction.
An alternative to shell like CMC structures is to orient the CMC limited laminated structures in a configuration so as to minimize the negative effects of anisotropy. In this configuration laminated structures are oriented so that the fiber ends are normal to the gas path surfaces thereby eliminating the concern of poor interlaminar properties. Such orientation is referred to as stacked laminated structures. Stacked laminate construction does however have some drawbacks. It results in higher raw material use and thus higher waste as compared to other construction methods. Intricate shaping of the component is possible using the stacked laminate construction but cutting to form the shape results in wasted ceramic fabric during the fabrication process. The contemporary cutting practices used in stacked laminate construction typically results in a component having a greater amount of total ceramic fiber content. Such wasted ceramic fiber during cutting and greater ceramic fiber contents in the components greatly increases the cost of turbine components made from stacked laminate construction. Due to the cost of the materials, there is often a trade-off between the cost of the component and the desired properties of the component, such as higher thermal conductivity or higher creep strength.
Thus, a need exists for construction methods and structures for laminated ceramic composite components having a lower cost. There is a further need for such components having improved properties, such as higher thermal conductivity or higher creep strength. In addition, a need exists for a ceramic article that may be used as a replacement material for metal parts in turbine systems to improve the efficiencies of the turbine systems.